Differential pressure transducer

ABSTRACT

A differential pressure measuring transducer is disclosed which includes a substantially rigid housing. The housing has two opposed end faces, a first centrally disposed cavity in a first end face, a second centrally disposed cavity in a second end face, and a passage connecting the two cavities. A first pressure responsive diaphragm is disposed adjacent the first end face and sealed thereto along its periphery. A second pressure responsive diaphragm is disposed adjacent the second end face and sealed thereto along its periphery. A rod extends through the passage and engages at each end a respective diaphragm. A first armature plate is attached to the rod adjacent the first diaphragm and a second armature plate is attached to the rod adjacent the second diaphragm. A first core is positioned in the first cavity contiguous with the housing and spaced from the first armature. The core has a centrally disposed opening therein in alignment with the passage, through which the rod may slide. A second core is positioned in the second cavity contiguous with the housing and spaced from the second armature. The second core also has a centrally disposed opening therein in alignment with the passage, through which the rod may slide. A first primary and secondary inductive coil assembly is mounted in the first core and a second primary and secondary inductive coil assembly is mounted in the second core so that each armature and core assembly constitutes a transformer. Means are provided for connecting the core assemblies into a variable inductive measuring circuit which includes primary energizing means. As a result, when the rod moves in response to differing pressures being sensed by the two diaphragms, one armature moves toward one core and the other armature moves away from the other core. Different output voltages are induced in each secondary coil with the difference between the two voltages being proportional to and a measure of the pressure differential being sensed by the diaphragms.

United States Patent Akeley [451 Aug. 1,1972

[54] DIFFERENTIAL PRESSURE TRANSDUCER Lloyd T. Akeley, Fullerton, Calif.

[73] Assignee: Beckman Instruments, Inc.

[22] Filed: Dec. 2, 1970 [21] Appl. No.: 94,400

[72] Inventor:

[52] US. Cl. ..73/398 R, 73/407 R, 336/30 511 lnt.Cl. ..G0ll9/10 [58]FieldofSearch ..73/4o7,39s R;336/30 [5 6] References Cited UNITED STATESPATENTS 3,534,612 10/1970 Buckland ..73/398 R Primary Examiner-Donald O.Woodiel Attorney-William F. McDonald and Robert J. Steinmeyer 5 7ABSTRACT second pressure responsive diaphragm is disposed adjacent thesecond end face and sealed thereto along its periphery. A rod extendsthrough the passage and engages at each end a respective diaphragm. Afirst armature plate is attached to the rod adjacent the first diaphragmand a second armature plate is attached to the rod adjacent the seconddiaphragm. A first core is positioned in the first cavity contiguouswith the housing and spaced from the first armature. The core has acentrally disposed opening therein in alignment with the passage,through which the rod may slide. A second core is positioned in thesecond cavity contiguous with the housing and spaced from the secondarmature. The second core also has a centrally disposed opening thereinin alignment with the passage, through which the rod may slide. A firstprimary and secondary inductive coil assembly is mounted in the firstcore and a second'primary and secondary inductive coil assembly ismounted in the second core so that each armature and core assemblyconstitutes a transformer. Means are provided for connecting the coreassemblies into a variable inductive measuring circuit which includesprimary energizing means. As a result, when the rod moves in response todiffering pressures being sensed by the two diaphragms, one armaturemoves toward one core and the other armature moves away from the othercore. Different output voltages are induced in each secondary coil withthe difference between the two voltages being proportional to and ameasure of the pressure differential being sensed by the diaphragms.

12 Claims, 4 Drawing Figures PATENTEDAusW I972 3.680.387

sum 1 or 3 FIG. 1

INVENTOR LLOYD T. AKELEY Admfwzmg/ PATENTEDAus 1 m2 SHEET 2 [IF 3 FIG. 2

PATENTEDMW I912 3.680.387

SHEET 3 BF 3 FIG. 3

68 S I HIGH S2 72 LOW 70 INVENTOR LLOYD T. AKELEY l DllFFERENTIALPRESSURE TRANSDUCER BACKGROUND OF THE INVENTION wherein the inducedvoltage or the difference in the induced voltage is the output of thetransducer.

Transducers or sensor gauges for differential pres-' sure measuringutilizing the inductive principle are known. The prior art variableinductance transducers have suffered from requiring either a pluralityof fill passages and internal and external diaphragm mounts or, in anattempt to overcome these problems, have resorted to a single centrallymounted sensing or measuring diaphragm. An example of such arrangementsmay be found in US. Pat. No. 3,277,719 to Prell. A typical earlier priorart approach may be found in US. Pat. No. 2,276,580 to Hofer.

- The central measuring diaphragm approach wherein a flat sensingdiaphragm deflects with pressure difference suffers because deflectionis not a linear function of pressure and because the induced change involtage is not necessarily a linear function of deflection. This limitsoperations to very small deflections if near linear performance isdesired. When diaphragm thickness is used to control stiffness, and thusdeflection, very close tolerances on thickness must be held. Also, thesensing diaphragm material must have a spring characteristic to minimizehysteresis and permanent offset with overrange.

SUMMARY OF THE INVENTION It is an object of the instant invention toprovide an improved differential pressure measuring transducer using avariable transformer or a variable inductance construction which doesnot depend upon a central measuring diaphragm. Advantageously, such atransducer will have improved linearity and reproducibility ofmeasurement. The assembly will be simplified and the number of openingsin the transducer housing for various connections will be substantiallyreduced.

The differential pressure measuring transducer of the instant inventionincludes a substantially rigid housing and two opposed end faces. Afirst centrally disposed cavity is in the first end face and a secondcentrally disposed cavity is in the second end face. A passage connectsthe two cavities. A first pressure responsive diaphragm is disposedadjacent the first end face and sealed thereto along its periphery. Asecond pressure responsive diaphragm is disposed adjacent the second endface and sealed thereto along its periphery. A rod extends through thepassage and engages at each end a respective diaphragm. A first armatureplate is attached to the rod adjacent the first diaphragm and a secondarmature plate is attached to the rod adjacent the second diaphragm. Afirst core is positioned in the first cavity contiguous with the housingand spaced from the first armature. The core has a centrally disposedopening therein in. alignment with the passage, through which the rodmay slide. A second core is positioned in the second cavity contiguouswith the housing and spaced from the second armature. The second corealso has a centrally disposed opening therein in alignment with th'epassage, through which the rod'may slide. A first primary and secondaryinductive coil assembly is mounted in the first coreand a second primaryand secondary inductive coil assembly is mounted in the second core sothat-each armature and core assembly constitutes a transformer. Meansare provided v for connecting the coil assemblies into a variableinductive measuring circuit including primary energizing means. When therod moves in response to differing pressures being sensed by the twodiaphragms, one'armature moves. toward one core and thefother armaturemoves away from the other core. Thus differing output voltages areinduced in each secondary coil, the difference between the two voltagesbeing a measure of the pressure differential being sensed by thediaphragms. i

If it is desired to fill the interior spaces of the transducer withfluid, the housing may have a fill passage therein for filling the spacebetween the diaphragms and respective end faces, the cavities, thepassage, the openings,'and any other hollow spaces within the housingwith fluid. Means can also be provided for sealing the fill passage.

Advantageously, a buffer disc may be provided between each armature andits respective core so that a cushion is provided between these partsunder overrange conditions. Each diaphragm and its respective end facemay also have matching contours with the diaphragm being designed toseat against the end face at a given pressure, whereby overrangeprotection is provided.

One end of the rod may engage one of the diaphragms by means of a capattached to a spring which resiliently bears against the correspondingend face of the housing. This may be advantageous in some circumstancesin that the spring will contribute to the stiffness of the system. Therange of the transducer can then be changed simply by changing springs.In this case a sleeve may be provided around the other end of the rod tomatch the magnetic leakage and loss in the spring, thus minimizingquadrature to provide good linearity and resolution through null. Meansmay also be provided for adjusting the length of the rod between thearrnatures so as to adjust the spacing between an armature and itsrespectigge core at zero differential pressure. In this way the zerooutput-can be precisely set.- The other end of the rod may engage theother diaphragm by means of a second cap. A shoulder may be provided oneach cap and a seat disposed on each end face for a shoulder to seatagainst under overrange conditions, whereby overrange protection isprovided.

Cross passages may be provided in the housing to facilitate electricalconnection between the coil assemblies and make possible the use of asingle four-pin header unit. A side-by-side construction of each coilassembly may be used to provide high impedance, primary-to-secondary,and coil-to-ground.

Desirably, the two secondary coils may be connected bucking in themeasuring circuit so that the output voltage from the instrument is thedifference between the voltages induced in each secondary coil.

BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages of thisinvention will be apparent from the following description taken inconjunction with the accompanying drawings.

FIG. 1 is a sectional view, with some parts removed for purposes ofclarity, of a differential pressure measuring transducer according tothe instant invention.

FIG. 2 is similar to FIG. 1, showing an alternate embodiment of adifferential pressure measuring transducer according to the instantinvention.

FIG. 3 is a connection diagram showing the manner in which the coils areconnected according to the instant invention.

FIG. 4 is a schematic diagram showing the wiring of the coils accordingto the instant invention.

DETAILED DESCRIPTION I Referring now to FIG. 1, the differentialpressure measuring transducer may be seen to include a substantiallyrigid housing indicated generally at 10. Housing may be of anyappropriate shape but conveniently is cylindrical. Housing 10 has twoopposed end faces, 12

and 14. A first centrally disposed cavity 16 is provided I in first endface 12 and a second centrally disposed cavity 18 is provided in secondend face 14. Passage 20 connects the two cavities. A first pressureresponsive diaphragm 22 is disposed adjacent first end face 12 andsealed thereto along its periphery in any appropriate manner, as bywelding. A second pressure responsive diaphragm is disposed adjacentsecond end face 14 and sealed thereto along its periphery, for examplein a similar manner. As shown, each diaphragm 22, 24 and its respectiveend face 12, 14, have matching contours and each diaphragm is designedto seat against its end face at a given pressure thus providingoverrange pro tection for the device. Rod 26 extends through passage 20and engages at each end a respective diaphragm 22, 24. As shown, end 28of rod 26 engages diaphragm 22 at first stiffening disc 30 by means of acap 32 attached to a spring 34 which resiliently bears against the endface 12 of housing 10 in first cavity 16. Diaphragm 22 is clampedbetween stiffening disc 30 and cap 32.

A first armature plate 36, for example of sintered iron powder, isattached to rod 26 adjacent first diaphragm 22. As shown, this isaccomplished through the first armature 26 being attached to cap 32.

The other end 38 of rod '26 is attached to second diaphragm 24 at secondstiffening disc 40 by means of a second cap 42. Sleeve 44 peripherallydisposed around second cavity 18 matches the magnetic leakage of spring34 to minimize quadrature in the output signal to provide goodresolution and linearity through null. A second armature plate 46,similar to first armature plate 36, is attached to rod 26 adjacentsecond diaphragm 24 through cap 42.

A first core 48 is positioned in first cavity 16 contiguous with housing10 at its inner face and spaced from first armature 36. First core 48has a centrally disposed opening 50 in alignment with passage 20,through which rod 26 may slide.

A second core 52 is similarly positioned in second cavity 18 contiguousat its inner face with housing 10 and spaced from second armature 46. Asecond opening 54 similar to first opening 50 is provided centrallydisposed in second core 52 in alignment with passage 20 trough which rod26 may slide. As shown, first and second buffer discs 56 and 58 of asuitable material such as polytetrafluoroethylene are positioned betweeneach armature 36, 46, and its respective core, 48, 52 so that a cushionis provided between these parts under overrange conditions. This willprevent possible chipping or fracture of the parts. A first primary 60and secondary 62 coil assembly is mounted in first core 48. A secondprimary 64 and secondary 66 inductive coil assembly is similarly mountedin the second core 52. Thus, each armature 36, 46, and its respectivecores, 46, 52 and coils 60, 62, and 64, 66, constitutes a transformer.The side-by-side coil assembly constructions shown, i.e. the primarycoil and secondary coil of each assembly are positioned side-by-side,provide high impedance, primary-to-secondary, and coil-to ground.

End 28 of rod 26 is shown threadably engaged with cap 32 to providemeans for adjusting the length of rod 26 between armatures'36 and 46 andtheir respective cores, 48, 52. The length of the rod would normally beadjusted during assembly of the transducer to precisely set the spacingbetween an armature and core assembly e.g., armature 46 and core 52, atzero differential pressure. Means such as electrical leads 68, 70, 72,74, 76,

and 78 are provided to connect the coil assemblies together into avariable inductance measuring circuit (not shown). Four electrical leads68, 70, 72, and 74 come out through housing 10 by means of a passage 80and pass through four-pin electrical header 82. A plug 84 at the end ofa conduit 86 threadably engaged with housing 10 supports header 82against internal pressure and due to the tapering of passage 80 sealsheader 82 to housing 10. Cross-passages 94 and 96 facilitate electricalconnection between the coil assemblies and make possible the use of asingle four-pin header unit 82.

A fill passage 88 is provided in housing 10 for filling the interiorspace thereof, i.e. the space between the diaphragms 22, 24, andrespective end faces 12, 14, the cavities, 16, 18, the passage 20, theopenings 50, 54, and all other hollow spaces, such as the interior partof passage 80, within the housing with an appropriate fluid such asinsulating oil. The fluid preferably is essentially noncompressible andhas the desired electrical insulating and dielectric properties andproper damping characteristics. Means such as a ball 90 pressure fittedinto tapered fill passage 88 may be utilized to seal fill passage 88when the interior of housing 10 has been filled. A second ball 92 may bestacked into fill passage 88 behind ball 90 to provide a backup force.

FIG. 2 is similar to FIG. 1, but shows an alternate arrangement forproviding overrange protection. Shoulders 98 and 100 are provided oncaps 32 and 42 respectively. A first seat, e.g. seal ring 102, isprovided on end face 12 and a second seat, e.g. seal ring 104, isprovided on end face 14. Under overrange conditions, the appropriateshoulder, 98 or 100, will seat against its respective seal ring, 102 and104, thus shutting off the flow of fluid from under its diaphragm, 22 or24, providing overrange protection.

Referring to FIGS. 3 and 4, the four windings of the two transformers ofthe transducer may be connected into any appropriate variabletransformer or variable inductance measuring circuit. A suitableelectrical power source not shown is connected across lead 68 connectedto the high side primary coil at S, and lead 70 connected to the lowside primary coil at F Lead 76 extends between F, of the high sideprimary coil 60 and S of the low side primary coil 64. The high sidesecondary coil 62 is connected to the low side secondary coil 66 by lead78 extending between F on the high side coil 62 and F on low side coil66. As shown, the two secondary coils 62 and 66 are connected bucking.Electrical lead 72 is connected to high side secondary 62 at S andelectrical lead 74 is connected to low side secondary 66 at 8,. FIG. 4is a schematic diagram of the 5 resulting circuitry.

Referring to FIGS. 3 and 4 it may be seen thatthere is a fixedexcitation from the primary coils 60 and 64. These produce amagnetomotive force'which, with a given gap between each armature 36,46, and core 48, 52 assembly, produces a given flux. As the gaps varydue to the movement of rod 26 in response to differing pressures beingsensed by the two diaphragms 22 and 24 causing one armature for example36 to move toward core 48 and the other armature 46 to move away fromcore 52, the flux in the two magnetic circuits varies and the voltage onthe secondary coils 62 and 66 varies. Since the secondary coils 62 and66 are connected bucking, the output voltage across lines 72 and 74 isequal to the difference in the induced voltages on the two secondarycoils 62 and 66. The change in output voltage is proportional to thechange in differential pressure. This voltage differential is a measureof the pressure differential being sensed by diaphragms 22 and 24. Anydifference in pressure on the two diaphragms 22 and 24 will cause thediaphragms to deflect toward the lower pressure.

It will be appreciated that the foregoing is a descripl. A differentialpressure measuring transducer comprising:

a. a substantially rigid housing having two opposed end faces, a firstcentrally disposed cavity in a first end face, a second centrallydisposed cavity in a second end face, and a passage connecting the twocavities;

a first pressure responsive diaphragm disposed adjacent the first endface and sealed thereto along its periphery;

. a second pressure responsive diaphragm disposed adjacent the secondend face and sealed thereto along its periphery;

a rod extending through the passage and engaging at each end arespective diaphragm;

e. a first armature plate attached to the rod adjacent the firstdiaphragm;

f. a second armature plate attached to the rod adjacent the seconddiaphragm;

a first core positioned in the first cavity contiguous with the housingand spaced from the first armature and having a centrally disposedopening therein in alignment with the passage, through which the rod mayslide;

. a second core positioned in the second cavity, contiguous with thehousing and spaced from the second armature and having a centrallydisposed opening therein is alignment with the passage, through whichthe rod may slide;

i. a first primary and secondary inductive coil assembly mou ted in thesecpnd core so that each armature an core assemb y constitutes atransformer; and k. means for connecting the coil assemblies into avariable inductance measuring circuit including primary energizing meansso that when the rod moves in response to differing pressures beingsensed by the two diaphragms, one armature moves toward one core and theother armature moves away from the other core, whereby differing outputvoltages are induced in each secondary coil, the difference between thetwo voltages being a measure of the pressure differential being sensedby the diaphragms.

2. The transducer of claim 1 wherein the housing has a fill passagetherein for filling the space between the diaphragms and theirrespective end faces, the cavities, the passage, the openings, and anyother hollow spaces within the housing with fluid.

3. The transducer of claim 2 including means for sealing the fillpassage.

4. The transducer of claim 1 including a buffer disc between eacharmature and its respective core so that a cushion is provided betweenthese parts under overrange conditions.

5. The transducer of claim 1 wherein one end of the rod engages onediaphragm by means of a cap attached to a spring which resiliently bearsagainst the corresponding end face of the housing.

6. The. transducer of claim 5 wherein a sleeve is peripherally disposedin the cavity around the-other end of the rod to match the magneticleakage of the spring, whereby quadrature in the output voltages isminimized.

7. The transducer of claim 6 wherein the other end of A the rod engagesthe other diaphragm by means of a second cap and including a shoulder oneach cap and a I seat disposed on each end face for a shoulder to seatagainst under overrange conditions, whereby overrange protection isprovided.

8. The transducer of claim 1 wherein each diaphragm and its respectiveend face have matching contours and the diaphragm is designed to seatagainst the end face at a given pressure, whereby overrange protectionis provided.

9. The transducer of claim 1 including means for adjusting the length ofthe rod between the armatures so as to adjust the spacing between anarmature and its respective core at zero differential pressure.

10. The transducer of claim 1 wherein the two secondary coils areconnected bucking in the measuring circuit whereby the output voltage isthe difference between the voltages induced in each secondary coil.

1 1. The transducer of claim 1 wherein cross-passages I are providedbetween the coil assemblies so that electrical connection isfacilitated.

12. The transducer of claim 1. wherein the primary coil and secondarycoil of each assembly are positioned side-by-side, whereby highimpedance is provided.

1. A differential pressure measuring transducer comprising: a. asubstantially rigid housing having two opposed end faces, a firstcentrally disposed cavity in a first end face, a second centrallydisposed cavity in a second end face, and a passage connecting the twocavities; b. a first pressure responsive diaphragm disposed adjacent thefirst end face and sealed thereto along its periphery; c. a secondpressure responsive diaphragm disposed adjacent the second end face andsealed thereto along its periphery; d. a rod extending through thepassage and engaging at each end a respective diaphragm; e. a firstarmature plate attached to the rod adjacent the first diaphragm; f. asecond armature plate attached to the rod adjacent the second diaphragm;g. a first core positioned in the first cavity contiguous with thehousing and spaced from the first armature and having a centrallydisposed opening therein in alignment with the passage, through whichthe rod may slide; h. a second core positioned in the second cavity,contiguous with the housing and spaced from the second armature andhaving a centrally disposed opening therein is alignment with thepassage, through which the rod may slide; i. a first primary andsecondary inductive coil assembly mounted in the first core; j. a secondprimary and secondary inductive coil assembly mounted in the second coreso that each armature and core assembly constitutes a transformer; andk. means for connecting the coil assemblies into a variable inductancemeasuring circuit including primary energizing means so that when therod moves in response to differing pressures being sensed by the twodiaphragms, one armature moves toward one core and the other armaturemoves away from the other core, whereby differing output voltages areinduced in each secondary coil, the difference between the two voltagesbeing a measure of the pressure differential being sensed by thediaphragms.
 2. The transducer of claim 1 wherein the housing has a fillpassage therein for filling the space between the diaphragms and theirrespective end faces, the cavities, the passage, the openings, and anyother hollow spaces within the housing with fluid.
 3. The transducer ofclaim 2 including means for sealing the fill passage.
 4. The transducerof claim 1 including a buffer disc between each armature and itsrespective core So that a cushion is provided between these parts underoverrange conditions.
 5. The transducer of claim 1 wherein one end ofthe rod engages one diaphragm by means of a cap attached to a springwhich resiliently bears against the corresponding end face of thehousing.
 6. The transducer of claim 5 wherein a sleeve is peripherallydisposed in the cavity around the other end of the rod to match themagnetic leakage of the spring, whereby quadrature in the outputvoltages is minimized.
 7. The transducer of claim 6 wherein the otherend of the rod engages the other diaphragm by means of a second cap andincluding a shoulder on each cap and a seat disposed on each end facefor a shoulder to seat against under overrange conditions, wherebyoverrange protection is provided.
 8. The transducer of claim 1 whereineach diaphragm and its respective end face have matching contours andthe diaphragm is designed to seat against the end face at a givenpressure, whereby overrange protection is provided.
 9. The transducer ofclaim 1 including means for adjusting the length of the rod between thearmatures so as to adjust the spacing between an armature and itsrespective core at zero differential pressure.
 10. The transducer ofclaim 1 wherein the two secondary coils are connected bucking in themeasuring circuit whereby the output voltage is the difference betweenthe voltages induced in each secondary coil.
 11. The transducer of claim1 wherein cross-passages are provided between the coil assemblies sothat electrical connection is facilitated.
 12. The transducer of claim 1wherein the primary coil and secondary coil of each assembly arepositioned side-by-side, whereby high impedance is provided.